Conventionally, an imager circuit comprises a matrix of pixels that are coupled to charge transfer circuits allowing the electric charge of the pixels to be transferred to a processing circuit after each exposure of the pixel.
The processing circuit may comprise a plurality of analog-digital converters allowing, for each column of the matrix of pixels, the charge value of the pixel to be converted, row by row, to a digital value.
The analog-digital conversion operation may be clocked using a single clock signal generator common to all of the analog-digital converters of the circuit. The clock signal is therefore propagated from the clock signal generator over a distance that is proportional to the size of the matrix of pixels, in order to be transmitted to the analog-digital converter associated with each column.
In order to obtain a high level of precision during the analog-digital conversion of the signal, it is preferable for the frequency of the oscillator to be as high as possible. Thus, doubling the frequency of the clock signal makes it possible to obtain a digital value coded on an additional bit.
As such, the higher the frequency of the signal, the more difficult it is to transmit it over long distances without this leading to a deformation of the signal.
FIG. 1 illustrates two timing diagrams c1 and c2 showing the variation in a clock signal of 800 MHz in frequency generated by a clock signal generator.
The first timing diagram c1 corresponds to a measurement of the signal taken directly at the output of the clock signal generator while the clock signal generator is not coupled to any propagation path. The duration t of the edges of the signal is short here, and shorter than quarter of the period of the signal.
The second timing diagram c2 corresponds to a measurement of the signal taken at the end of a propagation path of 4 millimeters.
It is observed that the signal is deformed.
Specifically, due in particular to the high capacitive load of the optical path, the duration of the edges of the signal is longer than quarter of the period of the signal. This deformation depends on the characteristics of the propagation path.
Conventionally, a follower amplifier is located at the end of the propagation line, making it possible to compensate for deformations of the signal. However, if the signal is deformed such that the duration of its edges is longer than quarter of the period of the signal, then the follower amplifier will no longer be capable of reshaping the signal, and the signal output by the follower amplifier will include distortions, such as, for example, high states and low states of different durations.
Thus, a signal is acceptable, i.e., considered to be non-deformed, if, for example, the duration of its edges at the input of the follower amplifier is shorter than quarter of the period of the signal.
It has been observed that the degree of deformation of the signal increases notably with the frequency of the propagated signal, and/or with the length of the propagation path.
It is therefore not currently possible to use a single high-frequency clock signal generator to clock all of the analog-digital converters of an imager once the size of its matrix of pixels becomes too large (typically a few millimeters).